The present invention generally relates to a system and a method for determining stretch or compression of a drill string. Sensors may be positioned along the drill string to obtain data related to stretch/compression of the drill string. The stretch/compression of the drill string may be used to calculate depths at which measurements are obtained by tools associated with the drill string.
To obtain hydrocarbons, a drilling tool is driven into the ground surface to create a borehole through which the hydrocarbons are extracted. Typically, a drill string is suspended within the borehole. The drill string has a drill bit at a lower end of the drill string. The drill string extends from the surface to the drill bit. The drill string has a bottom hole assembly (BHA) located proximate to the drill bit.
Drilling operations typically require monitoring to determine the trajectory of the borehole. Measurements of drilling conditions, such as, for example, drift of the drill bit, inclination and azimuth, may be necessary for determination of the trajectory of the borehole, especially for directional drilling. As a further example, the measurements of drilling conditions may be information regarding the borehole and/or a formation surrounding the borehole. The BHA may have tools that may generate and/or may obtain the measurements. The measurements may be used to predict downhole conditions and make decisions concerning drilling operations. Such decisions may involve well planning, well targeting, well completions, operating levels, production rates and other operations and/or conditions. Moreover, the measurements are typically used to determine when to drill new wells, re-complete existing wells, case wells, or alter wellbore production.
The tools obtain the measurements and associate the measurements with corresponding times. For example, a computer periodically calculates and records depths of the drill bit and associates a time with each depth of the drill bit. Thus, when the tools are retrieved from the borehole, the tools may transfer the measurements and the corresponding time data to the computer. The computer may use the times to associate the measurements with corresponding depths of the tools or sensors. The computer may generate a log of the measurements as a function of the depth of the drill bit.
Technology for transmitting information from the tools while the tools are located within the borehole, known as telemetry technology, is used to transmit the measurements from the tools of the BHA to the surface for analysis. At present, mud pulse telemetry is the only technique in widespread commercial use for communication while drilling, between downhole equipment and the surface (unless otherwise indicated, references herein to “while drilling” or the like are intended to mean that the drill string is in the borehole or partially in the borehole as part of an overall drilling operation including drilling, pausing, and/or tripping, and not necessarily that a drill bit is rotating).
In mud pulse telemetry, data is transmitted as pressure pulses in the drilling fluid. However, mud pulse telemetry has well-known limitations, including relatively slow communication, low data rates and marginal reliability. In many cases, this rate is insufficient to send all of the data that is gathered by an LWD tool string, or is limiting on the configuration of a desired tool string. Also, mud pulse technology does not work well in extended reach boreholes. Signaling from uphole to downhole by regulating mud pump flow, to control processes such as directional drilling and tool functions, is also slow and has a very low information rate. Also, under some circumstances, such as, for example, underbalanced drilling employing gases or foamed drilling fluid, current mud pulse telemetry cannot function.
There have been various attempts to develop alternatives to mud pulse telemetry that are faster, have higher data rates and do not require the presence of a particular type of drilling fluid. For example, acoustic telemetry which transmits acoustic waves through the drill string has been proposed. Data rates of acoustic telemetry are estimated to be approximately an order of magnitude higher than data rates of mud pulse telemetry, but are still limiting. Further, noise is a problem for acoustic telemetry. Another example is electromagnetic telemetry that uses electromagnetic waves transmitted through the earth. Electromagnetic telemetry is considered to have limited range and also has limited data rates. In addition, electromagnetic telemetry depends on characteristics, such as, for example, resistivity, of the formations surrounding the borehole.
The placement of wires in drill pipes for carrying signals has been proposed. Some early approaches to a wired drill string are disclosed in U.S. Pat. No. 4,126,848; U.S. Pat. No. 3,957,118; U.S. Pat. No. 3,807,502; and the publication “Four Different Systems Used for MWD,” W. J. McDonald, The Oil and Gas Journal, pages 115-124, Apr. 3, 1978.
The idea of using inductive couplers located at the pipe joints has also been proposed. The following disclose use of inductive couplers in a drill string: U.S. Pat. No. 4,605,268; Russian Federation published patent application 2140527, filed Dec. 18, 1997; Russian Federation published patent application 2040691, filed Feb. 14, 1992; and WO Publication 90/14497A2. Also see: U.S. Pat. No. 5,052,941; U.S. Pat. No. 4,806,928; U.S. Pat. No. 4,901,069; U.S. Pat. No. 5,531,592; U.S. Pat. No. 5,278,550; and U.S. Pat. No. 5,971,072.
U.S. Pat. Nos. 6,641,434 and 6,866,306 to Boyle et al., both assigned to the assignee of the present application and incorporated by reference in their entirety, describe a wired drill pipe joint that is a significant advance in the wired drill pipe art for reliably transmitting measurement data in high-data rates, bidirectionally, between a surface station and locations in the borehole. The '434 and '306 patents disclose a low-loss wired pipe joint in which conductive layers reduce signal energy losses over the length of the drill string by reducing resistive losses and flux losses at each inductive coupler. The wired pipe joint is robust in that the wired pipe joint remains operational in the presence of gaps in the conductive layer. Advances in the drill string telemetry art provide opportunity for innovation where prior shortcomings of range, speed, and data rate have previously been limiting on system performance.
More specifically, during the drilling phase of the construction of the wellbore, the length of the drill string in the borehole is used to estimate the depths or the along-hole lengths of a borehole based on an assumption that the drill pipe is inelastic and does not stretch. However, the assumption that the drill string is inelastic is not valid. The drill string stretches or compresses at various positions and is a function of several parameters, such as, for example, temperature, pressure and stress. The assumption that the drill string is inelastic may not yield sufficient accuracies for any number of reasons, such as formation testing or formation sampling.
Modeling, such as, for example, “torque and drag” modeling, attempts to compensate for the elasticity of the drill string. “Torque and drag” modeling is a complex modeling technique which involves modeling the interaction of the drill string and the borehole wall and modeling of bit behavior. Modeling is based on other assumptions regarding the drill string and the borehole that may lead to inaccuracies in data. For example, modeling does not account for friction on the individual pipe sections due to torturocity of the wellbore because the modeling is based on static surveys. Friction will translate into additional compressional forces on some pipe sections and not on other pipe sections even though these pipe sections may be adjacent to each other. Thus, the modeling will assign the same stress to both adjacent pipe sections even though the adjacent pipe sections may have different stress.